SUBIR PUNTO UNIÓN ENTRE LA BANCADA Y LA BARRA DOS

The experimental site was located at Carterton district of Wellington region, New Zealand. The current population of Carterton is close to 4200. There was a wastewater treatment plant that consists of fine screen, primary sedimentation tank with sludge digestion, and secondary and tertiary oxidations ponds at the time of experiment. The treated effluent was discharged to nearby Mangatarere stream. There was no history of applying treated effluent onto land before this experiment. At the time, the Carterton Council has the permission to discharge the treated effluent into the stream. The experimental site was very close to the oxidation ponds.

The soil was a stony silt loam from the recent fluvial deposits associated with the Mangaterere River and tributary streams. The land treatment area was located immediately adjacent to the oxidation ponds of the effluent treatment plant (Fig. 3.1). The study area comprised 0.162 ha, planted with either of two tree species Eucalyptus Nitens (E-N) and Eucalyptus Ovata (E-O)) or with pasture. At the commencement of the experiment the trees were approximately 2 years old.

There were three blocks, each of 540 m2 (Fig. 3.2). Each block consisted of 3 plots (2 plots of trees with the species in each plot randomly assigned and one plot of pasture). The area of each plot was 180 m2. The stocking density of each tree species was 5000 stems/ha. Previously, the land had been periodically grazed by sheep and had never been fertilised or irrigated with effluent (Carterton District Council staff). The topography within each irrigated plot was not leveled. Five monitoring wells were installed at the site – with one (wells 1, 2 and 3) in each effluent irrigation treatment

(Fig.3.2). The fourth well was installed in the pasture plot receiving the high effluent treatment, and the fifth (well 5) was installed on the upstream side of the experimental site, between the oxidation pond and plantation area (Figs. 3.1 and 3.2).

Fig. 3.2: A layout map showing the tree plantation and pasture areas, the locations of monitoring wells, and the areas receiving the low, medium and high rates of effluent application at the site.

The casings of all the monitoring wells were extended 1m above ground level, with the screened section starting 1m below ground level. Monitoring wells 3 and 4 on the downstream (river) side of the plantation extended only 3 m below ground level due to caving in of material during excavation, but the rest extended to a depth of 5 m. The location of these wells was determined on the basis of a piezometric survey conducted by Good Earth Matters Consultants.

W1 W2 W3 W5 Tree plots Pasture plots (Control) Low Medium High N O N O O N O N O N N O P P P O-Ovata N- Nitens P- Pasture W4 N

A trickle irrigation system was designed to apply sewage effluent at three hydraulic loading rates of 30, 45, and 100 mm per week (designated as low (L), medium (M), and high (H) treatments, respectively). The lowest hydraulic loading rate was chosen on the basis of N as the land limiting constituent (LLC). The hydraulic loading rate used for design is based on the more restrictive of two limiting conditions - the capacity of the soil profile to transmit water (soil permeability) or the limiting constituent concentration in the effluent applied. In municipal wastewater land treatment systems, N is usually the limiting constituent when protection of potable groundwater is a concern. If percolating water/effluent enters a potable groundwater, then the system should be designed such that the concentration of NO3-N in the receiving groundwater at the project boundary does not exceed 10 mg/L (WHO, 1984).

The concentration of total N in the sewage effluent was 15.5 g/m3, and the maximum allowable N loading rate according to the Wellington Regional Council Rule 11 (Annual Wellington Regional Council Plans, 1997, pp. 65) was 150 kg N/ha/year. The designed N loading rates for the low, medium, and the high irrigation treatments were 150, 225 and 525 kg N/ha/year, respectively.

The trial began in December 1997 and continued through to August 1998. Monitoring of site specific factors relevant to the climate, effluent, soil, plants, and groundwater was undertaken.

The daily rainfall, and maximum and minimum daily temperatures were measured at the site. Daily evaporation was not measured at the site. Instead, daily pan evaporation data recorded at Martinborough and crop (i.e. pasture) evapotranspiration (ET) data recorded at East Taratahi were collected from the National Institute of Water & Atmospheric Research, New Zealand. The two sites were on different sides, but close (approximately 10 km) to the experimental site.

The daily pan evaporation data recorded at the Martinborough station was used to calculate the potential evapotranspiration (PET) for pasture and tree crops for the experimental site. A crop factor of 0.75 was selected for pasture. This figure was chosen after noting that crop factors in agriculture typically range from 0.6 to 1.0 (e.g. Israelsen and Hansen, 1962; Stewart et al., 1988). The crop factor was then multiplied by the pan evaporation data to estimate the pasture PET. The results of the estimated pasture PET (from Martinborough) and pasture ET (from East Taratahi station) were compared. A t-test was carried out to show the statistical comparison between the ET values from both stations.

Because of limited data on the water use of natural Eucalyptus Nitens and Ovata stands, and less information on water used by trees under effluent-irrigated conditions, a final crop factor of 1.5 for both tree species was selected to calculate the tree PET. This was based on evidence that forest uses more water than grassland (e.g. Holmes and Colville, 1970; Stewart et al., 1988).

A bi-weekly climatic water balance approach was adopted to estimate the actual evapotranspiration (AET) as reported by Myers et al. (1994). The input parameters used in the climatic water balance were; precipitation (PPT) measured at the site, PET estimated for the site, and the potential soil water storage (SMS) which can be calculated by multiplying the water holding capacity (WHC - mm/m) of the soil by the effective plant root depth (m).

The soil texture at the site was silt loam and therefore a WHC of 200 mm/m was assumed (Department of Agricultural Engineering, 1983). The effective root depths for pasture and trees were assumed to be 0.6 m and 1.0 m respectively (Bryan Myers, personal communication, 1999). Similar root depths are also reported by Myers et al. (1994).

The calculated soil water storage values for pasture and trees (i.e. 120 mm and 200 mm) were used in the climatic water balance in order to determine the AET for both crops (pasture and trees). The estimated AET values were used in the water balance to calculate the drainage losses in all treatments on the tree and pasture plots. The data on drainage losses are reported in Chapter 4. A Kent flow meter was used to measure the amount of effluent added to the system during each irrigation cycle. Flow meter readings were taken to check that the system was running at the designed application rates.

Volumetric soil moisture content (SMC) measurements were made using Time Domain Reflectometry (TDR) probes at three depth ranges (0 - 150, 0 - 300, and 0 - 500 mm) prior to irrigation, and then fortnightly in all treatments on the tree and pasture areas. Soil temperature measurements were made using soil temperature probes at soil depths of 150, 300 and 500 mm prior to irrigation, and then fortnightly.

Soil-water samples were collected from suction cups. These porous ceramic cups, 25 mm in diameter and 60 mm long, were installed at depths of 150, 300 and 500 mm in all plots (pasture and tree) by boring holes in the soil to the required depth, and then placing sand around the ceramic cups, and filling the holes with the original soil from the site. Two suction cups were installed for each depth per tree plot per irrigation treatment – giving a total of 36 suction cups in the tree plots. One suction cup was installed for each depth per pasture plot per irrigation treatment – giving a total of 9 suction cups in the pasture plots.

The effluent, soil-water and groundwater samples were collected prior to irrigation and then fortnightly. Fifteen soil core samples were collected at the beginning of the experiment from each tree and pasture plots for 0 - 500 mm depth using an electric soil core sampler. Three soils samples were taken from each soil core at 150, 300 and 500 mm depth (i.e. 15 samples for each soil depth). Another fifteen samples were collected at the conclusion of the experiment using the same method, and the soil samples were

collected at the respective depths from each core. The samples collected before and at the conclusion of the experiment were analysed separately for each depth. The effluent, soil pore water, and groundwater samples were stored at 4oC and then analysed for total Kjeldahl nitrogen (TKN), NO3-N and NH4-N using the standard methods of water analysis (Gillian, 1984). Soil total N and available N (i.e. NO3-N and NH4-N) were determined using the standard Auto Analyser method (Kamphake et al., 1967 and Gillian, 1984) and expressed on a dry weight basis for the 150, 300 and 500 mm depths in all the tree plantation and pasture treatment areas, before and at the conclusion of the experiment. The total and available N contents in the top 500 mm of soil were calculated by multiplying the soil bulk density at different depths with the N concentration at those depths, and then adding the values for all depths up to 500 mm. Soil bulk density, soil pH, and soil organic carbon were determined for the 150, 300 and 500 mm depths in all the tree plantation and pasture treatment areas before and at the conclusion of the experiment. The soil bulk density and pH samples were all bulked. A survey was carried out to determine the levels of the bed of the effluent channel (which was carrying the tertiary treated effluent from the oxidation pond to the river), the river bed, and the ground surface levels of well 1, well 2 and well 3.

Four trees of each species were selected from each tree plot, around the suction cup and TDR probe sampling points. Height and diameter at breast height (DBH) measurements were taken prior to and at the end of experiment. In each plot the diameter of selected trees was measured 15 cm above the ground. This is called the basal diameter. Trunk wood (stem wood) sub-samples of both tree species were collected at the end of experiment when the trees were almost three years old. Tree leaf samples were collected from the selected trees in each tree plot before and at the end of the experiment.

The pasture was cut at a height of 10 mm before irrigation and removed from the site. The next pasture cut was made to the same height 10 weeks after the first irrigation, then the final pasture cut was made at the end of the experiment to determine the total dry matter production of pasture (kg/ha/38 weeks). The leaf and pasture samples

collected before and at the end of the experiment, and stem-wood sub samples collected at the conclusion of irrigation were analyzed for N using the standard methods of plant analysis (Gillian, 1984).